(a) Field of the Invention
The present invention relates to a gray-scale representation method for plasma display panels (PDPs). More specifically, the present invention relates to a gray-scale representation method for PDPs that includes determining the number of sustain pulses for each subfield in consideration of address light.
(b) Description of the Related Art
The PDP is a display device that has a plurality of discharge cells arranged in a matrix form that are selectively excited to emit light and thereby to reconstitute image data originally input as electrical signals.
Gray-scale representation must be achieved on the PDP so as to represent the performance of the PDP as a color display device. A gray-scale representation method divides one field into a plurality of subfields and subjects the subfields to time division control to achieve gray-scale representation by subfields.
Each subfield is largely divided into an address period and a sustain period. The address period is for sending data for each pixel to the respective sustain and address electrodes to selectively discharge the individual cells or erase them. The sustain period is for representing gray scale while maintaining the data of each pixel.
Among these methods, the most general method for representing gray scale on PDPs is the ADS (Address Display Separated) method developed by a Japanese company, Fujitsu, that completely separate the address period from the sustain period.
In addition, gray scale related PDP patents are disclosed in U.S. Pat. Nos. 5,835,072, 6,294,875B1 and 6,353,423B1.
The ADS method involves controlling the amount of light for sustain solely to achieve gray-scale representation on PDPs. Namely, the subfield weight as determined by the number of sustain pulses is fixed, or one field is divided into 10 to 12 variable subfields according to the APC (Automatic Power Control) level determined by the load ratio of an image to represent 0 to 255 on a gray scale of 255.
FIG. 1 shows a frame structure in the conventional ADS method.
In the conventional PDP, as illustrated in FIG. 1, reset, address and sustain periods constitute one subfield, and a combination of several subfields forms one frame according to the ADS method.
The light from one subfield is the sum of light emitted during the discharges of address and sustain. Typically, gray scale is represented by a combination of light emitted only during the sustain periods. This is because the amount of the reset or address light is insignificant relative to that of the sustain light.
Recently, development of the PDP technologies has had a tendency to minimize PDP cell dimensions by an increase of the Xenon (Xe) partial pressure and fineness for achieving high brightness, thereby realizing High Definition (HD) PDPs in the true sense of the word, and to change the partition wall structure from the conventional stripe type to a closed type. This reflects the tendency to develop PDPs of high efficiency, high brightness, and fineness.
The tendency to increase the Xe partial pressure, achieve the fineness of cells, and change the partition wall structure to a closed type results in an increase in the amount of light emitted during the address discharge, so that the resultant address light becomes too significant to ignore in the gray-scale representation.
FIG. 2(a) shows, for example, the weight and the number of sustain pulses by subfields in the conventional PDP, and FIG. 2(b) shows the subfield structure by gray scales represented with a combination of the weights by subfields of FIG. 2(a) and the resultant light structure. FIG. 3 is an illustration of light emitted for the subfield in a general PDP.
As shown in FIGS. 2(a) and 2(b), one frame is comprised of 12 subfields, the sum of the subfield weights is 255, and the total number of sustain pulses is 511. In the figures, the symbol “A” denotes the sum of reset light and address light.
Accordingly, the light emitted for one subfield can be expressed by the following equation:
                                                                                             one                  ⁢                                                                          ⁢                  subfield                  ⁢                                                                          ⁢                  light                                =                                ⁢                                                      reset                    ⁢                                                                                  ⁢                    light                                    +                                      address                    ⁢                                                                                  ⁢                    light                                    +                                                                                                                        ⁢                                  sustain                  ⁢                                                                          ⁢                  light                                                                                                        =                                ⁢                                  A                  +                                      the                    ⁢                                                                                  ⁢                    number                    ⁢                                                                                  ⁢                    of                    ⁢                                                                                  ⁢                    sustain                    ⁢                                                                                  ⁢                    pulses                                                                                                            [                      Equation            ⁢                                                  ⁢            1                    ]                    
It is assumed that the light emitted from one sustain pulse is unit luminescence 1.
Under this subfield weight, the subfield structure of gray scale 1 corresponds to 3SF, that is, the third subfield, and the resultant light structure is A+3. For a gray scale of 6, the subfield structure corresponds to 1SF, 2SF and 3SF, that is, the first, second, and third subfields, and the resultant light structure is 3A+15. For a gray scale of 7, the subfield structure is 3SF and 4SF, that is, the third and fourth subfields, and the resultant light structure is 2A+16.
As described previously, the conventional gray-scale representation is a combination of subfields that only depend on the number of sustain pulses. This can be achieved when the reset light or the address light represented by A is insignificant relative to the sustain light. When A is insignificant, for example, the number of sustain pulses is 15 for a gray scale of 6, and 16 for a gray scale of 7. The gray scale of 7, in this case, has more sustain pulses than the gray scale of 6 and hence the larger total amount of light for subfields to achieve more correct gray-scale representation and higher brightness.
Here, the reset light is not so significant. But, when the address light is equal to or greater than the sustain light, the brightness for the gray scale of 6 becomes equal to or greater than that for the gray scale of 7, as a result of which correct gray-scale representation is difficult to achieve.
More specifically, in the above example, the light structure is 3A+15 for the gray scale of 6 and 2A+16 for the gray scale of 7. When the address light is equal to or greater than the unit sustain light, i.e., A≧1, the difference between the gray scale of 6 and the gray scale of 7 is given by the following equation:(3A+15)−(2A+16)=A−1≧0  [Equation 2]
It can be seen from the equation 2 that the brightness for the gray scale of 6 is equal to or greater than that for the gray scale of 7 to achieve incorrect gray-scale representation when the address light is equal to or greater than the unit sustain light.
Accordingly, the tendency to increase the Xe partial pressure, achieve the fineness of cells, and change the partition wall structure to a closed type for realizing high brightness results in an increase in the amount of light emitted during the address discharge, and the resultant address light becomes too significant to ignore in the gray-scale representation, as a consequence of which correct gray-scale representation is difficult to achieve.